The court, presided over by Judge Luis Alaez, has not released the conductor's name but he has been widely identified in the media as Antonio Martin Marugan.

On Thursday, the judge said the conductor, who had called the driver to discuss which track to use on a later stop, was not being accused of criminal negligence.

"The fact of consulting the driver to know if the train could run on a particular track was something normal," the judge said. "It was not the cause of the derailment."

"Even if it was unfortunate that the call took place at that place and time, it is not sufficient to make an accusation of criminal negligence."

The train driver, 52-year-old Francisco Jose Garzon Amo, has been released on bail charged with 79 counts of reckless homicide while the court investigates.

With 79 people killed and more than 100 injured, it was Spain's worst rail disaster since 1944.

Garzon had said in his first testimony to the Galicia regional court on Sunday that he "didn't understand" how he failed to brake in time, a recording of his court hearing revealed.

"I can't explain. I still don't understand," the driver told the judge when asked why he hadn't slowed down in time to take a sharp bend four kilometres (three miles) away from Santiago de Compostela.

Asked again about what caused him to crash, he added: "I tell you sincerely that I don't know. Otherwise I would not have been so crazy as not to brake" earlier.

Railway officials say the track where the train crashed was not equipped with the automatic braking systems in place on some high-speed lines and that it was therefore left up to the driver to brake.

The driver told the judge he had braked, but by the time he did so the crash was "inevitable".

"Before the train turned over, I had activated everything but I saw that no, no, it wasn't working."

The black box data recorders revealed the train was going at 192 kilometres (119 miles) per hour before braking shortly before the bend. When it derailed it was travelling at 153 kph -- nearly twice the 80 kph speed limit on that part of the line.